What is a sign of a stable air mass?

A sign of a stable air mass is indeed characterized by persistent stratus clouds and fog. In a stable air mass, the atmosphere resists vertical motion, which leads to the development of these low, layered clouds. Stratus clouds form when warm, moist air gently rises and cools, resulting in uniform cloud cover that can extend over large areas, often accompanied by fog when the humidity is high. This typically indicates that there are no significant temperature changes to promote turbulence or convective activity.

In contrast, other options suggest instability in the air mass. Thermals causing turbulence are associated with rising air and unstable conditions. Cumulonimbus clouds with thunderstorms indicate strong vertical development typical of unstable air, leading to convective activity. Varied cloud formations suggest a mix of different atmospheric conditions, which is also indicative of instability. Thus, the presence of persistent stratus clouds and fog is a clear indicator of a stable air mass.